Those several apps probably don't do what Photoshop Touch does. As mentioned before, the common denominator is tablet hardware, specifically memory (most high-end tablets top out at 1 GB of RAM).
If you want more megapixels, prepare to exchange it in terms of features or performance until tablets catch up with their desktop brethren.
Take a look at filterstorm.
It does almost everything photo shop does. It can cope with up to 22mp on the ipad 2.
Okay so it crashes a lot and only manages 6 layers, but 22mp
Surely you could manage 11?
Laminar will do 3872x3872 (15MP) and has no set limit for layers. I really want to like Photoshop Touch but seriously there are plenty of cheaper apps that can cope with much higher res images and layers for a lot less money. Yes, I can import a hi res image to work on while I am on the move but I can't export it back to Photoshop without losing image quality so what is the point. If they need to lose more layers to get the res up then so be it.
Pravnav Kapoor (the developer of Laminar) lists this little tidbit:
Laminar is needs space in the physical disk in order for allow it to work. Space is only temporarily used in order to save the undo states, etc. Please ensure that the device has at least 256MB of free space for the software to work correctly, and preferably around 500MB if working with images larger than 10MP.
Half a gig of (I'm assuming flash memory) space just to edit anything larger than 10 megapixels. Not sure how much memory PS Touch uses at once on an image but I'm sure it depends on the image size, number of layers, etc.
PS Touch does now work with about 5 megapixel images with aplomb on my Motorola Xoom (1 GB of memory; 32 GB of space). This actually now matches my Xoom's camera resolution.
My point still stands. If you want features rivaling that of the desktop expect a hit elsewhere, at least until tablets catch up.